1. Field of the Invention
The invention relates to a control apparatus that switches operation states of two disconnect mechanisms in accordance with a traveling state of a four wheel drive (4WD) vehicle. The 4WD vehicle is provided with two disconnection mechanisms that are disposed on a driving force source side and an auxiliary driving wheels side of apparatus a driving force transmission shaft, respectively, as the disconnect mechanisms.
2. Description of Related Art
4WD vehicles that are provided with two disconnection mechanisms as disconnect mechanisms are widely known. The two disconnection mechanisms are disposed on a driving force source side and an auxiliary driving wheels side of a driving force transmission shaft (for example, propeller shaft), respectively, the driving force transmission shaft transmitting power of the driving force source during 4WD traveling to the auxiliary driving wheels side becoming driven wheels during two wheel drive (2WD) traveling. One of the examples is a vehicle that is described in Japanese Patent Application Publication No. 2010-100280 (JP 2010-100280 A). In this 4WD vehicle, the two disconnection mechanisms are released during 2WD traveling, and thus the rotation of the driving force transmission shaft and the like between the two disconnection mechanisms can be stopped. In this case, fuel efficiency can be improved compared to a case where only one of the two disconnection mechanisms is released during 2WD traveling. In addition, various techniques have been proposed with regard to control for transition from 2WD traveling to 4WD traveling in the 4WD vehicle. JP 2010-100280 A discloses a technique in which, for example, control is performed so that a torque-variable multi-plate clutch (corresponding to one of the two disconnection mechanisms) is engaged in a case where the main driving wheels slip during 2WD traveling so that the rotation speed of a torque transmission section (corresponding to the propeller shaft) is increased, the relative rotation speed of the dog clutch (corresponding to the other one of the two disconnection mechanisms) is substantially synchronized, and the dog clutch is engaged. In addition, JP 2010-100280 A discloses a technique for accelerating the torque transmission section so that the acceleration of the vehicle does not exceed an acceleration limit value rarely felt by a driver by the acceleration of the torque transmission section. In addition, it is disclosed that this acceleration limit value is set in advance depending on environmental factors such as road surface conditions, a wheel slip value, a pedal position, and a steering wheel position.